Abstract. The Critical Zone (CZ) is a holistic framework for
integrated studies of water with soil, rock, air, and biotic resources in the near-surface terrestrial environment. This most
heterogeneous and complex region of the Earth ranges from
the vegetation top to the aquifer bottom, with a highly variable thickness globally and a yet-to-be clearly defined lower
boundary of active water cycle. Interfaces among different
compartments in the CZ are critical, which provide fertile
ground for interdisciplinary research. The reconciliation of
coupled geological and biological cycles (vastly different in
space and time scales) is essential to understanding the complexity and evolution of the CZ. Irreversible evolution, coupled cycling, interactive layers, and hierarchical heterogeneity are the characteristics of the CZ, suggesting that forcing, coupling, interfacing, and scaling are grand challenges
for advancing CZ science. Hydropedology – the science of
the behaviour and distribution of soil-water interactions in
contact with mineral and biological materials in the CZ –
is an important contributor to CZ study. The pedosphere is
the foundation of the CZ, which represents a geomembrance
across which water and solutes, as well as energy, gases,
solids, and organisms are actively exchanged with the atmosphere, biosphere, hydrosphere, and lithosphere, thereby
creating a life-sustaining environment. Hydropedology emphasizes in situ soils in the landscape setting, where distinct
pedogenic features and soil-landscape relationships are essential to understanding interactive pedologic and hydrologic
processes. Both CZ science and hydropedology embrace
an evolutionary and holistic worldview, which offers stimulating opportunities through steps such as integrated systems approach, evolutionary mapping-monitoring-modeling
framework, and fostering a global alliance. Our capability to
Correspondence to: H. Lin
(henrylin@psu.edu)

predict the behaviour and evolution of the CZ in response to
changing environment can be significantly improved if crosssite scientific comparisons, evolutionary treatment of organized complex systems, and deeper insights into the CZ can
be made.

1

Introduction

“Our own civilization is now being tested in regard to its
management of water as well as soil.” Daniel Hillel (1991)
The US National Research Council (NRC, 2001) recommended the integrated study of the “Critical Zone” (CZ) as
one of the most compelling research areas in Earth sciences
in the 21st century. This CZ is defined by the NRC (2001) as
“a heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air and living
organisms regulate the natural habitat and determine availability of life sustaining resources.” This zone ranges from
the top of the vegetation down to the bottom of the aquifer
(Fig. 1). It encompasses the near-surface biosphere and atmosphere, the entire pedosphere, and the surface and nearsurface portion of the hydrosphere and lithosphere (Fig. 2).
The US National Science Foundation (NSF AC-ERE, 2005)
also recommended a focus on water as a unifying theme
for understanding complex environmental systems. Waterrelated research requires enhanced understanding of processes at environmental interfaces, approaches for integrating across scales, and improved coupling of biological and
physical processes. Collectively, such an integrated, interdisciplinary, and multiscale effort will advance our ability to
forecast and plan for changes and to address critical societal
issues such as human safety, human health, economic prosperity, environmental quality, and sustainable development.

Published by Copernicus Publications on behalf of the European Geosciences Union.

26

H. Lin: Earth’s Critical Zone and hydropedology

Fig. 1. Concepts of the Critical Zone, regolith, weathering profile, soil profile, and solum (modified from Schaetzl and Anderson, 2005).
The unmodified/unweathered portion of the C horizon is labelled as D horizon (after Tandarich et al., 1994). All materials above fresh,
unweathered bedrock (R horizon) are called regolith, which is equivalent to a broad definition of the soil. The Critical Zone is the broadest
concept, going from the top of the tree to the bottom of the aquifer. Also illustrated on the right are (1) the abundance of microbes and insects
in the soil, especially in the surface soil, and (2) fresh water seeping out of an aquifer and running over exposed bedrock (modified from
anonymous sources).

Soil is at the central junction of the CZ, representing
a geomembrance across which water and solutes, as well
as energy, gases, solids, and organisms are actively exchanged with the atmosphere, biosphere, hydrosphere, and
lithosphere, thereby creating a life-sustaining environment
(Fig. 2). Water is the circulating force that drives many of
these exchanges and is the major transport agent in the cycling of mass and energy in the CZ. Water flux into and
through the soil and over the landscape is the essence of life,
which resembles the way that blood circulates in a human
body (Bouma, 2006). The interactions of soil and water are
so intimate and complex that they cannot be effectively studied in a piecemeal manner; rather, they require a systems
and multiscale approach. In this spirit, hydropedology has
emerged in recent years as an intertwined branch of soil science and hydrology that addresses the interface between the
pedosphere and the hydrosphere, with an emphasis on in situ
soils in the landscape setting (Lin, 2003; Lin et al., 2006a).
The pedosphere is a unique, relatively immobile sphere
that is easily impacted by human activities. In contrast to the
other spheres of the Earth system, the pedosphere can neither
Hydrol. Earth Syst. Sci., 14, 25–45, 2010

quickly intermix (as the atmosphere does), nor rapidly move
laterally along the landscape (as water does), nor clearly be
separated into individual units and avoid undesirable environmental changes (as the biota can be and does), nor escape rapid human and biological perturbations (as is characteristic of the lithosphere). Therefore, each soil, as a relatively immovable and formed in situ natural body, is fated
to react, endure, and record environmental changes by being transformed according to the interactions of climatic,
biotic, and anthropogenic forcing, as conditioned by geologic and topographic setting, over geological and biological time scales. This makes the monitoring of soil change
an excellent (albeit complex) environmental assessment, because every block of soil is a timed “memory” of the past and
present biosphere-geosphere dynamics (Arnold et al., 1990).
This memory takes multiple forms, such as micromorphology, profile features, and various soil physical, chemical, and
biological properties. Learning to “decode” soil features and
their changes into environmental information is as valuable
as reading the records of ice cores for atmospheric conditions
and interpreting tree rings for eco-climatic dynamics.

www.hydrol-earth-syst-sci.net/14/25/2010/

In 1962.
which has been recently reviewed by McClain et al.
Lohse et al.” As of 15 March 2009. 2010
. Cameron called
a geological formation (the Bushveld Complex in South
Africa) a “critical zone. Tsakalotos (1909). time..hydrol-earth-syst-sci. 25–45. (3) a highlight of
fundamental characteristics and research opportunities of the
CZ and hydropedology.H.
2
2. (3) Soil is a gas and energy regulating
geoderma in the land-atmosphere interface. (5) Soil is a living porous substrate essential for
plant growth. The “7+1” roles of the soil from the Earth’s Critical Zone to Mars exploration (modified from Lin. and food supply. and (8) Soil is a possible habitat for extraterrestrial life
– if any can be found – and serves as a frontier in extraterrestrial explorations for signs of liquid water and life. (4) Soil is the foundation of diverse ecosystems.
and related efforts in establishing environmental observatories around the world. (2) Soil is a fresh water storage and transmitting mantle in the Earth’s Critical Zone.
(7) Soil is a great natural remediation and buffering medium in the environment. hydropedology. two
models for the outlook of soil science are illustrated: one is a broad perspective (“outward” growth) and the other is a narrow view (“inward”
contraction).
Given the emerging interests in the CZ. and others. and human
impacts. Earth Syst. E. relief.1
Critical Zone science
History of the concept of the Critical Zone and its
current meaning and utility
The generic term of “critical zone” first appeared a century
ago in a German article by physical chemist D. and synergistic efforts. multiscale. 2. 2005): (1) Soil is a natural recorder
of the Earth’s history through its formation and evolution under the influence of climate. and (4) a discussion on possible
ways for advancing CZ science and hydropedology. (2) a suggestion of a broadened perspective for future soil science advancement. this article reviews and discusses
growing opportunities for synergistically advancing soil science.net/14/25/2010/
disciplines involved in CZ studies) to illustrate the opportunity and the need for integrated. and geosciences. hydrology. (2009). (6) Soil is a popular material for a variety of engineering and construction applications. a total of 314 publications
Hydrol. This discussion is timely
because soil science programs worldwide have struggled to
survive. parent material. using hydropedology as an example (among many other
www. This
paper focuses on physical and hydrologic aspects of the
CZ. Specific objectives
of this paper include: (1) a clarification of the basic ideas
of the CZ and hydropedology. Sci. based on the
Science Citation Index Expanded. the hydrology community is becoming united because of the current global concern for fresh water. and
the geosciences community has embraced an expanded vision of its role and societal relevance. In the lower panel. N. American mineralogist E. Coupling physical/hydrologic and biogeochemical/ecological processes in the CZ is an emerging topic. and their relations to relevant disciplines. Lin: Earth’s Critical Zone and hydropedology
27
Soils
Critical
Zone
Atmosphere
Mars Exploration
(3)
(8)
Deep vadose zone
Water table
Critical Zone
Biosphere
Pedosphere
(Soils)
(4)
Ground water zone
Hydrosphere
(2)
Agriculture
Environment
(5)
Lithosphere
(6)
(1)
Broad
perspective
(outward
growth)
(7)
Engineering
Critical Zone
Critical Zone
Soils
Soils
Narrow
perspective
(inward
contraction)
Fig. (2003). animal production. referring to the zone of a binary mixture of two
fluids. 14. organisms.

a pedologist (Larry Wilding). personal
communication. But the diffuse lower boundary
of the CZ may extend to a kilometer or more beneath
a fractured bedrock and the volume of water stored in
this zone may be an order of magnitude larger than the
combined volume of water in all rivers and lakes (NRC.. Many people thought that the CZ is (nearly) the same
as soils.
land-ocean interface. 2).
this is precisely the benefit for using the CZ concept as
it will force us to better define the variable and dynamic
lower boundary of the active water cycle. including plants and animals). 25–45. life processes (macro.hydrol-earth-syst-sci. generally <1–2 m deep. soil formation and evolution (pedogenesis). the geochemical cycle. loess. 2 (Lin.
4. the CZ is much broader than soils
(see Figs. Some pressing scientific issues involved include global climate change. An attempt is
thus made here to clarify some related issues or concepts:
1. Regolith is synonymous with
mantle or overburden. 14. life-water-mineral interactions.
1998). while focusing on
soil processes. 2009).
In 2001. and human
impacts (land use and management).
2006. 1987). thereby permitting an
inclusive vision for future soil science. see Fig. The CZ. It is true that the CZ encompasses
the entire pedosphere – the only sphere in the Earth system that is wholly included in the CZ. Sci.
3. This CZ concept was suggested
by a subgroup of the NRC Committee on Basic Research
Opportunities in the Earth Sciences (L. 2). This “outward”
growth perspective is consistent with the soil’s recognized “7+1” roles as depicted in Fig. 2001).
2. gas exchange (major and trace gases).net/14/25/2010/
. 1). and a hydrologist (Stephen Burges). As indicated by the NRC (2001). Lin: Earth’s Critical Zone and hydropedology
C horizon is often called saprolite. Currently we do not
know exactly where the active water flow ceases in the
subsurface in different ecosystems and geographic regions.. and is equivalent to the broader
concept of soil that includes O-A-E-B-C horizons (the
Hydrol. Since then. especially under unsaturated flow conditions. 2008). Instead.28
have used the term “critical zone” to refer to wide-ranging
things: from geological formation where precious metals
(such as platinum and gold) can be mined (Wilhelm et al. In reality. 1). Some have used the term CZ as synonymous with a
common geological term “regolith” (defined as “the
fragmental and unconsolidated rock material..
1997) to pipe corrosion in soil within ground water fluctuation zone (Decker et al. 1990) to local cold
regions of ice-structure interaction where intense pressures
occur over short time leading to ice failure (Johnston et al. weathering (chemical and physical). glacial
drift. 2001). the carbon cycle.
1991. a broadened perspective of soils is needed (this is labeled here
as “outward” growth. 1). yet such an understanding is important as this
demarcation influences the annual. Bouma.
terrestrial carbon cycle.. and century hydrologic cycles. and 15 papers referred to water related issues. Some people have questioned the utility of the CZ concept because of its imprecise lower boundary and highly
variable thickness from place to place. 2010
H. 2006). Many researchers believe that the CZ concept is useful
because it is inherently process-oriented and is a unifying framework that accommodates the hydrologic cycle.
integrates regolith with above-regolith vegetation and
below-regolith bedrock or sediments that interact with
fluctuating ground water (Fig. Earth Syst. 2001). On the contrary. alluvium. The
classical narrower concept of soil has been driven by
agriculture-centric conception of the soil as a medium
for plant growth. the NRC specifically defined the “Critical Zone”
(note capitalized letters).. which consisted of a sedimentologist
(Gail Ashley). The timescales included in the CZ concept range from seconds to eons
and its spatial scales are enormous – from atomic to
global (NRC. 2005) and
seven soil functions identified by the EU Soil Protection
Strategy (Commission of the European Communities. that nearly everywhere forms
the surface of the land and overlies the bedrock. vegetal accumulations. enthusiasm as well as
skepticism have surfaced in scientific communities regarding the meaning and utility of this concept. The majority of these 314 papers were in the subject areas of geosciences (46%) and minerals/energy (41%). the nutrient cycle. then it loses the integrating and unifying power of the CZ.and microbial
communities. if the
CZ is limited to just soils (we label this perspective as
“inward” contraction. can then see their knowledge in a much
broader context. see Fig. and soil. Hydrologic and biogeochemical models have often been forced to make assumptions about the lower boundary of the active water cycle
(such as an impermeable bedrock or an artificial 2-m
cut-off for soil depth). 1 and 2). see
Fig. see Fig. erosion and deposition. which often only includes the A-B
horizons (called solum. 2008).
www. 2001) to
transitional zones in alluvial coastal plain rivers important
for water resource management (Phillips and Slattery..
from body ventricular slow conduction area with electrophysiological limitations (Elsherif et al. decadal. whether
residual or transported. from the rhizosphere where
soil and roots have close interaction (Ryan et al.
the rapidly expanding needs for a sustainable society
give special urgency to understanding the processes that
operate within the complex CZ.
Only 15 of them used the term of “critical zone” in relation
to soils. It includes rock debris of all kinds – volcanic ash. Soil scientists. and earth history (NRC. lithification (diagenesis). as defined by the NRC (2001).”
Bates and Jackson. However. Wilding.

Lin: Earth’s Critical Zone and hydropedology
29
Biological Feedback & Cycle
Life System Evolution
Solar
radiation
Entropy
(S)
Evolution – Accumulation
Entropy & Info Flow
Info
(I)
Time
Evolution
(forcing)
Sources
Forces
Critical
Zone
E.. M. 14. The interfaces between belowground layers are critical controls
of the CZ’s response and feedback times to aboveground changes such as climate and land use.
www. 2010
. however. The CZ has contrasting layers vertically. some general
characteristics of the CZ can be identified (Fig. The CZ is highly heterogeneous horizontally. and hierarchical
heterogeneity are the characteristics of the CZ. Together. 3).. Sci. Cycles of geological
and biological processes over vastly different spatial
and temporal scales are a key to understanding the CZ. The reconciliation of geological and biological cycles vastly different in space and time is
essential to the understanding of the complexity and dynamics of the CZ. S. interactive layers. The CZ is a coupled system. Increasingly.2
General characteristics of the Critical Zone
The CZ is perhaps “the most heterogeneous and complex region of the Earth” (NRC.H. Schematic of the Critical Zone (CZ) that is open to continuous energy and mass exchange with the surrounding environment. I
Vertical
Layers
(interfacing)
Earth’s
gravity &
internal
energy
Cycles
(coupling)
Energy
(E)
Sinks
Fluxes
Horizontal
Heterogeneity
(scaling)
Space
Energy & Mass Flow
Conservation – Balance
Mass
(M)
Landscape Evolution
Geological Feedback & Cycle
Fig. S. which
dictate the evolutionary outcome and the functioning of the CZ. human alternations
to the CZ have become pervasive and long-lasting. coupled cycling.hydrol-earth-syst-sci. The CZ is evolutionary.2.1
Irreversible evolution
Natural systems are open systems in terms of energy. Ubiquitous heterogeneity in the CZ exhibit both hierarchical
organizations (which exhibit gaps between conceptualized scales) and networks embedded in land mosaics
(which provide efficient transfer of mass and energy
across scales). Changes in the CZ are generally irreversible and cumulative. Earth Syst. which
include:
1. abrupt changes. where
the conservation of energy (E) and mass (M) and the accumulation of entropy (S) and information (I ) are at work simultaneously. Irreversible evolution. 3. 2001) and the soil has been recognized as “the most complicated biomaterials on the planet”
(Young and Crawford. which may be introduced by either slow. more gradual processes or extreme.
Each of the above four general characteristics is further
elaborated in the following. instead of the
classical reductionistic and mechanistic worldview.
2. Such thermodynamically open and dissipative
systems have driven the evolution and functioning of the CZ
Hydrol.
2. NRC. I
E. because solar energy enters freely and heat energy goes back
into space. Despite such
extreme complexity and dynamics. 2009). M. these can help forge a
holistic and evolutionary worldview of the CZ. 25–45.
which involves the coupling of different spheres of the
Earth system. 2004.net/14/25/2010/
3.
4.
2.

2006). make the evolution of
life possible (Jørgensen. transport. 2010b). rather they are legion and can occur everywhere all
the time and at all scales. or irregular and unpredictable outcomes.hydrol-earth-syst-sci. Brantley
et al. along with information stored and
aesthetics (quality) accumulated over time. Therefore. slippage
back towards it (through dissipation) (Ulanowicz. no
matter how numerous and daring human technological reorganizations or transformations of matters may become. Many
physical. force capable of reshaping the face of the
planet” (Clark et al. weathering. 2007). As dictated by the second law of thermodynamics. Such a common phenomenon is
probably linked to the irreversible nature of time and dissipative processes described above. 2010
H. complex
structures are formed through the system’s self-organization
(a process of attraction and repulsion in which the internal organization of a system increases in complexity without being
guided by an outside source).
Time irreversibility is essential to the evolution of the CZ
(Lin. evolution progresses towards more and more complex and organized systems (Tiezzi.2. The
only exceptions are the slow escape of lightweight gases
(such as hydrogen) from the atmosphere into space and the
input of frequent but tiny meteors and cosmic and meteoric
dusts (Christopherson. 2009). Soil thickness in the CZ. for
example. biological laws that govern the
structure and function of the CZ (Lin. 1971. a multiplicity of states. A balancing act exists between
these two opposing tendencies in nature: moving away from
thermodynamic equilibrium (via organization) vs. because energy
dissipation results in heat lose that cannot be totally converted back to work should the process be reversed.
1896). instead.
The CZ has two overarching cycles that are vastly different in space and time scales: the geological cycle (dubbed
“big” cycle) and the biological cycle (dubbed “small” cycle)
(Fig. the erosion. water. the Earth’s physical materials are finite (for all practical purposes) and hence tend to
depend on cycling for continued use. play a fundamental role in natural evolutionary processes (Tiezzi. the properties of which can be quite different from those of the underlying elementary laws and may
take the form of abrupt transitions. 2006. and many ordered systems exhibit emergent properties (i.30
(Lin. 2010b).
and then are lithified and uplifted back to the land by tectonic
or volcanic activities. e. and other resources – the Earth is essentially a closed system from a
global perspective.e. occurring over the geological timescale and often a large area. Earth Syst. rather.
Increasingly.. This implies that. It is the reconciliation of these two cycles that is
critical to understanding the CZ’s complex evolution and dynamic functions. nature has a tendency to go toward disorder.. and deposition of weathered products that eventually end up in oceans as sediments. is composed of three major sub-cycles – tectonic. Prigogine’s (1977) theory of dissipative structures suggests that. 3) (Lin.
This duality leads to complex systems that disobey linear
deterministic laws.
The tectonic cycle brings heat energy and new materials to
www. 2010b). Such an imbalance between the slow production
of new soils (which is generally infertile subsoil above fresh
bedrock) and the accelerated lose of fertile topsoil has severe
negative and irreversible consequences for the sustainability
of the CZ. This is the essence of life. 2004)..net/14/25/2010/
. Richter and Mobley. 2006). according to Boltzmann. Unlike space. Consequently. and potentially intelligent. with
spontaneous increase in entropy (which can be interpreted
as an index of a system’s disorder. Tiezzi. Lin: Earth’s Critical Zone and hydropedology
selection involved in evolution gives rise to unexpected structures and events. 4). Besides the forcing of tectonics. 2007. Both
energy and mass conservation and entropy and information
accumulation are at work simultaneously in the evolution of
the CZ (Fig. the combination of chance and
Hydrol.. 2004. In the meantime. 25–45.
The CZ is also awash in events that occur once and never
again (Lin. Furthermore. at least at the biological timescale. Anthropocene
is now recognized as “a new geological epoch in which humankind has emerged as a globally significant.2
Coupled cycling
In terms of physical matters – soil. This cycle. (2007) and Ulanowicz (2009) suggested that singular events in nature are not
rare. Jørgensen et al. chemical. illustrates the accelerated changes caused by humans: average rate of soil formation from bedrock weathering has been estimated by many as 1 mm per 1000 years or
less. mechanisms to store information gained. fluid transport. the
material base of the CZ is fixed and limited. 1977). properties that a system possesses in addition to the sum of its components’ properties.g. In particular.
2006). 1995. which
is referred to as deterministic chaos (Nicolis. Rinaldo et al.. cross fertilization
between evolutionary and conservative principles is essential
to understanding physical vs. 2010b). Thus. life is only possible if it does not start from zero
for every new generation. 4). 2009). 2010b). the duality
of chance and choice is characteristic of evolution of natural
systems (Monod.
Dissipative structure...
Indeed. and hydrologic (Fig.
the human forcing is increasingly recognized. air. the exact same conditions can not occur again in
the natural world (Tiezzi. 2006). that is. Over time.
new patterns. Sci. 2006).
2. 14. lithologic. as energy dissipates. while the rate of soil erosion has reached an average rate
of 10–20 mm per 1000 years in many areas around the globe
because of human activities (McKenzie et al. time cannot be reversed because
time always moves forward in one direction only. Tiezzi. through genes. Time
evolution also displays accumulative and memory effects. 2006). 2006). 2006).
and biological activities as highlighted by the NRC (2001). living creatures and ecosystems obey the laws of biological evolution:
at any time they are different from what they were an instant before (Tiezzi. the
whole is more than the sum of its parts). and biological changes occurred in nature are also irreversible (Prigogine. human impacts on the CZ have become fundamental (Richter. The geological cycle refers to the weathering of rocks.

and has the dampening effect of state
variables with depth and an increase in distance to energy input at the soil surface.Biological Cycle
le
yc
lc
ca
mi
he
oc
ge
Bio
Hy
dr o
log
ic
cy
cle
Ecological cycle
cle
cy
gic
olo
dr
Hy
Ro
ck
-so
il c
yc
le
H. creating movement and
deformation of the crust. and trophic relations determine the nature of an ecosystem. traditional discipline-limited and individual
component-based efforts have plagued our understanding of
the coupled CZ system. 25–45. there is an overall trend
of increasing characteristic response time (i. the cycling of nutrients.net/14/25/2010/
31
Fluid transport is involved in both the geological and biological cycles. 1990). redistributes elements and materials through liquid (e.. and gas (e. At the biological
timescale. the belowground root zone. and further down to the
lithosphere (Arnold et al. Earth Syst. 2000). which are integrated into the evolutionary history of the Earth and life.
2. Many computer models. 2010
. mathematical models have been used to describe
and predict the distribution of important elements of interest. Using the most basic description of the cyclic
processes. with each layer
having various sub-layers. At this
geological timescale. The geological time diagram
in the background is courtesy of G. The biosphere is ultimately
what ties the major systems of the Earth together and drives
them far out of thermodynamic equilibrium (Jacobson et al. The ecological (or life) cycle generates biomass through producers such as microbes and plants.3
Interactive layers
The Earth is a vertically layered system: from the outer atmosphere down to the inner core. 1 and 2).. water). 2000). especially those of
the global scale.g. Life on Earth as we know it would be impossible without liquid water.g. down to the
hydrosphere.. since water is the key conduit for mass and
energy transfer. 1). biogeochemical. the time period
needed to feedback to climate and land use changes generally
Hydrol. carbon.e. Sci. time period
needed to reach a quasi-steady state with the environment)
to external perturbations from the atmosphere. different layers of materials
with vastly different thicknesses and characteristics are evident (Figs. This allows an element budget to be defined
for the entire cycle. however. ice. The rock (or rock-soil) cycle produces igneous. This layering has a general trend
of increasing density with depth (largely because of gravity. Principle sub-cycles within the geological “big” cycle and
the biological “small” cycle.2.
but with exceptions). However.
2000). sediments). alternatively.
the surface and recycles materials..
such as water. pedosphere. the
amounts of materials in each reservoir are assumed to be at
steady-state. Therefore.
solid (e. Lin: Earth’s Critical Zone and hydropedology
Tectonic cycle
Geological Cycle
Fig. In many cases. and sedimentary rocks in the
crust. metamorphic. sediment.
reaching consumers and eventually detritivores through the
food chain.. and others (Jacobson et al.
and wind. including the soil cycle as rocks weather into soils and
soils return to rocks over the geological timescale. human activities can be elevated
to a separate cycle because of their increasingly dominant
impacts on the biological cycle.
The biological cycle refers to the production and consumption of food and energy in the ecosystem and the accumulation and decomposition of organic materials in soils.. Soils support vast communities of microorganisms that decompose organic matter and re-circulate elements in the biosphere (see an illustration in Fig. has limitations because it provides little or
no insight into what goes on inside each reservoir or the nature of the fluxes between reservoirs (Jacobson et al. 4).
www. The cyclic approach mimics the coupled nature of
the CZ.. Such a steady-state. and hydrologic cycles (Fig. 4. the details
of the distribution of an element within each of the reservoirs
are disregarded. The CZ consists of the aboveground
vegetation zone. Conversely. and for the most simplified calculations. air). the hydrologic cycle transports organic and inorganic materials and energy throughout the CZ.
The average-based analysis also does not consider spatial and
temporal variation.. use this approach. Ashley of Rutgers Univ. biosphere.g.. The
flow of energy.g. 14. It is therefore hoped that the emerging CZ science can significantly advance the holistic and evolutionary worldview of complex coupled systems such as the
CZ. e. budget-based approach. biogeochemical cycle is inseparable from the
hydrologic cycle. nitrogen. the deeper vadose zone. combining biotic and abiotic processes. Anthropologic impacts may be considered as part of the ecological cycle.hydrol-earth-syst-sci. the biological cycle occurs over a much
shorter timescale and often a smaller area. The biogeochemical (or elemental) cycle.
The cyclic approach allows conceptual simplification of
materials movement and their couplings to the environmental factors.. Three principle
sub-cycles involved are ecological. Compared to
the geological cycle. and therefore could give a false impression of certainty. the hydrologic cycle processes materials with the physical and chemical actions of water. and together they determine the ecological
cycle. and the saturated aquifer zone.

Interfaces
between soil horizons with different textures or structures
also often trigger preferential flow. 1999):
– Inherent processes: the substrate heterogeneity beneath
the land mosaic is dependent on the processes of geology.32
decreases when moving up from the deep underground zone
to the shallow subsurface zone to the surface zone and to the
aboveground zone. or redox features
at the surfaces of soil particles or aggregates. These interfaces are unique and important controls of the landscape-soil-water-ecosystem-climate
relationships. 25–45. and interference with the climate and biota. 2010
H.2. transport.g. hurricanes. Differentiation
of systematic vs. 2008). fluctuating ground water has
a significant influence on the nature and properties of the
lower part of soil profiles (Jenny. urbanization. Bell (1999) suggested that it
is possible to consider landscapes as complexes of networks
and mosaics.hydrol-earth-syst-sci.
Within a soil profile. adsorption/desorption.
However.
Changing ground water levels also has effects on the energy
balance at the soil surface and the susceptibility of a region
to drought (Maxwell and Kollet.e. Sci. Interfaces
between soil layers often slow down vertical water percolation and promote lateral flow.
Patterns in the landscape.
Hydrol. soil chemical reactions generally occur at various
interfaces. geomorphology.. organisms. the soil-stream interface. Heterogeneity here differs from randomness: the former is associated with order while the later
is linked to disorder (Lin. and reaction processes.. are often unclear because they comprise many layers of elements. The NRC (2004)
highlighted the importance of ground water fluxes across interfaces when estimating recharge and discharge to aquifers. such as clay films. and subsurface. the
macropore-matrix interface. many challenges remain in understanding and
measuring dynamic interchanges among the water reservoirs
of atmosphere. Mosaic patterns can be found over a wide range of spatial scales (e. such as the meandering and branching systems that
run through and between the elements that produce the mosaics. and temporal inference (or prediction) of diverse processes occurred in
the CZ. and yet the upper few centimeters of
sediments beneath nearly all surface water bodies (i. and diseases) to
the biota that colonize and grow on the variable substrate. b).. Currently no single universal theory has emerged that is ideal for spatial aggregation
(or upscaling). and parent material). (1998) showed convincing evidence that terrestrial ecosystems significantly interact
with atmospheric processes. oxidation/reduction. Lin: Earth’s Critical Zone and hydropedology
2. i. whereas
random (disordered) variation is stochastic.e. and thus they are as important
as changes in atmospheric dynamics and composition. with each element having its own heterogeneity. Soil-forming theory also offers a useful framework for understanding diverse mechanisms leading to subsurface heterogeneity. Sophocleous (2002) reviewed recent advances
in ground water and surface water interactions. and hydrology.
The mosaic patterns can be determined by mechanisms characteristic of underlying processes. leading
to structural and compositional heterogeneity at all scales. where systematic (ordered)
variation can be identified by environmental controls (such as
climate. 2010a. the vadose zone-ground water interface. Such interfaces are also important in defining at what depth a soil
profile will begin to saturate and at what time of a year. 1941. ice sheet extent. all
of which are places where important interactions occur. especially with the
subsurface.e. and possible ways of bridging scales
can be further explored. carbonates.
– Anthropomorphic processes: human activities ranging
from land use/land cover changes to modifications of
landform. because patterns of heterogeneity are the diagrams of
processes (Bell. These elements are often
intricately woven together due to the interactions of different
processes occurred in the CZ. Pielke et al. pedology.. including the equilibrium and kinetic processes
of dissolution/precipitation.
Key interfaces in the CZ include the land surfaceatmosphere interface. Narasimhan.
the water-air interface. and polymerization/biodegradation. Earth Syst. 2005). hyporheic zone) have a profound effect on the chemistry of
water interchanges. and orbit perturbations in the
studies of past and future climate change. important interfaces also exist. giving rise to various runoff patterns in catchments (Lin et al. surface. For example. the surface water-ground
water interface.net/14/25/2010/
. ocean
circulation. including the soil horizons interface. however.. random variations allows a focus on the
portion of the CZ variability that can be related to known
causes.
www. For
instance. the microbe-aggregate interface.
2008b). especially in sloping landscape with an underlying water-restricting layer. insect pests. certain causes of heterogeneity in the
CZ can be identified. including (Bell. from the submicroscopic soil matrix to the
entire pedosphere). the soil-root interface.
– Extrinsic processes: the effect of natural disturbances
(such as fires. a dissipative system in nature is characterized by
the spontaneous appearance of symmetry breaking (leading
to anisotropy) and the formation of complex structures (leading to heterogeneity). Another
example is illuviation that leads to various coatings on soil
interfaces. and pointed
out that most models today are not well equipped to deal
with local phenomena related to flow near domain boundaries (i. interacting with the climate and biota. the soil-vegetation interface. interfaces). and the soil-water table interface..
Heterogeneity is expected based on Prigogine’s (1977)
theory. and thus can impact the
scaling of flow. and the soilbedrock interface. 14. Despite this. landform. The networks are patterns of linear-oriented
features.4
Hierarchical heterogeneity
Ubiquitous heterogeneity and scale bridging underlie nearly
all of the CZ studies. disaggregation (or downscaling). 1999). Mosaics arise because of uneven and dynamic energy inputs into the open system of the CZ.

2010
. a quantitative and operational hierarchy that can
be integrated into models of scaling.. Randomness (or structureless) is the property that makes statistics work here. Organized complexity
(systems)
→ Evolutionary treatment?
I. .
As remote sensing techniques for monitoring large-area phenomena and in situ sensors for point-based measurements
continue to advance. This is the region of systems with medium numbers.
3. 5): this is the region of large populations. The
dominant process and controlling factor may also change in
a hierarchical manner as scale changes. spectroscopic...
However. where systems of medium numbers are too complex for analysis and
too organized for statistics. within which the two classical methods (analytical and statistical treatments) essentially fail. (b) Three types of systems with respect to different methods of thinking (modified
from Weinberg.. and
computer modeling coupled with remote sensing and other
techniques are increasingly used at the global level.
Von Bertalanffy (1968) indicated that there is a general
lack of appropriate scientific means for dealing with systems
between extremes – the systems of so-called medium numbers (Fig.
There is an apparent hierarchy in the heterogeneity of soils
and the CZ (Fig. where gaps exist
in between hierarchical levels (Fig. emergent properties) from the complex interactions of
a large number of different elements at a smaller scale. and other technologies become widely used at the pore or even molecular levels. 5). 2005). from the pedon to the hillslope and
catchment scales. Note gaps exist between scales. Organized
simplicity
(machines)
→ Analytical treatment
Complexity
Fig. and i±1. flow.
Pedon at a local point is considered here as the basic scale (i) of observation or monitoring. 5): this is the region too complex for analysis and
too organized for statistics. 25–45. 5):
this is the region of machines. and hydrologists as a means for organizing natural
systems from the pore scale up to the global scale (Fig. Unorganized complexity
(aggregates)
Upscaling
Zoom out
Hillslope
processes
Mesoscopic
Pedon
i+1
i
Profile
processes
i-1
Microscopic
Aggregate
processes
i-2
Downscaling
Mineral-organic
complex processes
i-3
Pore & molecular processes
i-4
Zoom in
→ Statistical treatment
Randomness
Macroscopic
III. 5. Therefore hierarchical frameworks have been utilized by geoscientists.e. and rate processes
remains a major research challenge today. Systems of “unorganized complexity” (Region II in
Fig. or mechanisms. bridging multiple scales from points to
watersheds and to the entire globe becomes essential. 5). Sci. Systems of “organized simplicity” (Region I in Fig. Patterns may emerge at a large scale
(i. While
advanced imaging. Note the wide gap between the two extremes. soil
scientists. 2002). Lin: Earth’s Critical Zone and hydropedology
33
(a) Hierarchical Scales
(b) Three Types of Systems
Global processes
i+4
Regional processes
i+3
Watershed
processes
i+2
II. 5) becomes a significant gap that is
often neglected and has only limited and often rudimentary tools and techniques for investigating the most complex and dynamic subsurface. Weinberg (1975) described three types of systems that require different thinking:
1. 14. . 4 indicate arbitrary labels of larger
(+) or smaller (−) scales.hydrol-earth-syst-sci.net/14/25/2010/
2.
www. the intermediate scale (e. Fig. (a) A hierarchical framework for bridging multiple scales from the molecular to the global levels (modified from Lin et al. Earth Syst.
but yet sufficiently random so that it is sufficiently regular to be studied statistically. This region has small populations with a great deal of structure. 5). This region is complex.g. This intermediate scale is also
Hydrol.H. where
statistical mechanics applies. The present inability
to adequately characterize subsurface heterogeneity exacerbates the scaling problem and leads to significant uncertainties in data interpretations and model predictions for the CZ
across scales (Sophocleous. 1975). 5). Systems of “organized complexity” (Region III in
Fig.
which could be treated analytically.

and aggregate stability). down hillslopes (101 –103 m). and
size distribution of peds. residence time (age of water). and seeks to answer
the following two basic questions (Lin et al. with each
horizon structured to various degrees (Fig.
microbe-aggregate. and the macropore-matrix. which in return
reinforce or modify the existing networks. tortuosity.
3
Hydropedology
3.. 6) (Lin. While hydrogeology. and functions?
Landscape water here encompasses the source. during hydrological events
100 –102 h. such as coatings on peds or pores... mycorrhizal
mycelial networks. How does landscape water (and the associated transport of energy. In the following.
Fundamental scientific issues of hydropedology.
density. Lin (2010a) also showed that networks are abundant in soils.
Hydrol. only a brief
update is provided to exemplify the contributions from hydropedology to the needed integrated. How do soil architecture and the distribution of soils
over the landscape exert a first-order control on hydrologic processes (and associated biogeochemical and
ecological dynamics) across spatio-temporal scales?
2. artificial subsurface drainage networks. preferential flow can operate during fluid flows at
the temporal order of 100 –101 s. including
soil matrix (represented by soil texture and soil microfabic)
and soil aggregation (represented by the type. and nonlinear dynamics of hydrologic processes..hydrol-earth-syst-sci.34
H. Sci. connectivity. 6). pathway. McDonnell et al. Lin: Earth’s Critical Zone and hydropedology
essentially the region of medium number systems that urgently needs a new way of thinking and a significantly improved scientific treatment (Lin. 2010a). variability in
energy and mass inputs to soils. Lin (2010a) has attempted to justify
the likely universality of preferential flow in natural soils –
meaning that the potential for preferential flow occurrence
is everywhere in nature. diversity in biological activities. an important missing piece of the puzzle is
the interface between the hydrosphere and the pedosphere.1
Fundamentals of hydropedology
Hydrogeosciences have encountered a new intellectual
paradigm that emphasizes connections between the hydrosphere and other components of the Earth system. and across
inter-annual variations of 101 years. and
pore networks between soil particles and aggregates. (2008)
summarized. attention to flow pathways (especially flow networks). such as root branching networks.
Hydropedology addresses this interface.1. quantity. Soil morphology and pedogenesis vs. including the size distribution. 2005):
1.
www. at this
point of its development.
Based on three theoretical considerations and numerous
published evidence. As Clothier et al. and (3) interfaces
between solid components and the pore space. chemicals.
Because of heterogeneous soil architecture. and flux of water have been studied extensively in the past.net/14/25/2010/
. and biomaterials
by flowing water) influence soil genesis. Soil functions and maps vs. animal borrowing networks. hydrometeorology. Earth Syst. through
catchments (104 –105 m). and spatiotemporal distribution of water in the variably unsaturated or
saturated near-surface environment (Lin et al. 2010b).
in pedons (100 m). 14. Soil catena and distribution pattern vs. and ecohydrology are now
well recognized. and horizons interfaces. variability. 2008a.. b):
1. water movement
over the landscape: Hydropedology focuses on quantitative relationships between soils and their surrounding
landscape and the impacts of such relationships on hydrologic (and related biogeochemical/ecological) processes. although the actual occurrence of
preferential flow depends on local conditions (Lin and Zhou. (2) pore
space. While
source. 25–45. Soil architecture is the entirety of how the soil is structured.
3. interdisciplinary. which encompasses at least three parts: (1) solid components. and across large regions of ≥106 m.1
Soil structure and horizonation: their impacts on
preferential flow
Natural soils generally contain multiple horizons.
availability. evolution.
2006a. soil-root. 2007). residence time. throughout seasonal changes 100 year. soil hydrology
and soil change: Hydropedology emphasizes quantitative soil hydromorphology as a signature of soil hydrology and the soil as valuable records of environmental
change over time. in situ water flow and
chemical transport: Hydropedology emphasizes soil architecture (solid + pore + their interfaces) and its quantitative links to preferential flow across scales. 2006a). “carriers” of soil quality and
soil-landscape heterogeneity: Hydropedology promotes
quantitative delineation of functional soil units in the
landscape as well as precision soil-landscape mapping
for diverse applications. storage.
2. These
networks provide preferential flow conduits. and morphology of various pores. may be considered under the following four headings (Lin et al.
2008). availability. (2005) have provided a comprehensive review
on these fundamental issues. at the core scale (10−1 m). sediment. and
multiscale studies of the CZ. Soil structure and horizonation vs. preferential flow can occur in practically all natural soils and
landscapes (Fig. crack and
fissure networks.
Time-wise. 2010
4. flux.. storage.
3. and
spatio-temporal pattern of flow dynamics (and its underlying organizing principle) has been much limited (Lin et al. preferential flow can occur spatially at the pore
scale of spatial order 10−3 m.
Lin et al.

.
www. and
(2) At the regional and global scales..g. 6.
2005. and geomorphic processes across spatial and temporal scales. the
pedologic portion of a landscape (Hole. 2006b.
There are also strong links between ubiquitous heterogeneity and diverse preferential flow in natural soils and landscapes.. 14.g. zonal soil patterns can
be expressed by a gradual change in soils over large areas.
2003). climate.
3... 7b the topography of the underlying weathered rock substrates with low-permeability causes subsurface distribution
patterns of soil and hydrology (Coventry. 2006. 1982). Lin et al. The lower right
diagram of a hillslope is modified from Atkinson (1978). Such
a soil pattern is often referred to as a “soilscape”. Wilcox et al. which includes catenae and other more localized soil
distribution patterns (such as gleization and Histosols). hydrologic. solutes. soil patterns are heterogeneous mainly due to factors
that vary over short distances such as topography and parent materials (i. water table depth. 2006b). 7). Lin et al. A key to hydropedologic study is the connection between the pedon and landscape paradigms.
2008b).. water. 1985. and sediments typically differ in soils along
a catena (Fig..e. 2010
. 7a vs. Hydropedology as an interdisciplinary science that promotes the integrated studies of hydrologic. Schaetzl and Anderson.. 2002. and pedologic approaches
– is the most promising way forward. Sci.. the site factors of soil formation). the flux
factors of soil formation). The soilbedrock interface and bedrock topography have now been
recognized as important to subsurface stormflow in many
hillslopes (e.. and land use. Drainage condition. Lin et al. 25–45. b) may occur depending on the
local controls of surface or subsurface topography. and fluxes of
water. Earth Syst.. contrasting hydrology and soil
morphology (e.net/14/25/2010/
Catenae in different climatic and physiographic regions
may exhibit markedly different relationships between soil
and hydrologic properties (e. whereas in
Fig.. Figure 7a
can be explained by the classical catenary model where surface topography controls hydrological regimes. Tromp Van Meerveld and
McDonnell.e.g.H.
Hydrol.2
Soil catena and distribution: their controls on
landscape hydrology
There appear two general categories of soil distribution patterns that can be differentiated in terms of their main controls at different scales: (1) At the hillslope and landscape
scales.1. Fig.
2006).
resulting from climatic and vegetative gradients (i. i. These physiographically-oriented
soil patterns are recognized in the US as Major Land Resources Areas (which are defined as geographically associated land resource units that are characterized by a particular
pattern of soils. pedologic. Lin: Earth’s Critical Zone and hydropedology
35
Hydrologic Processes
(Water)
The Pedon
Paradigm
The Landscape
Paradigm
Pedologic Processes
(Soil)
Restrictive layer
(perched water table)
Geomorphic Processes
(Landscape)
Precipitation
Depression
-focused flow
Critical
Zone
Fig.e. USDA-NRCS. However. Sommer and Schlichting (1997) suggested that an integration of three different approaches to study soil distribution over the landscape – geomorphic/stratigraphic.hydrol-earth-syst-sci. Buol et al. Freer et al.. 2007.

8a
illustrates a general sequence of soil development from a
weakly-developed Entisol to a highly-weathered Ultisol under well-drained condition: as soil age increases. S.g.. 2004. In particular. 2008b). which has
potentials in quantifying pedogenesis. Water from precipitation is a primary requisite for
parent material weathering and soil development. but saturated hydraulic conductivity in the subsoil (e.e.and micro-morphology have long been used to
infer soil moisture regimes. and promote plant and animal health
(SSSA. Fig. whereas the downslope deep Red Kandosols have
much greater depth to free water (4–11+ m). 1993). 2005). an argillic horizon in an Ultisol may
turn into an aquitard. sufficient amount of water must
not only enter the profile and participate in weathering reactions. with shallow depth to free
water (0–2 m).3
Soil morphology and pedogenesis: their records of
soil hydrologic change
Soil macro.1.. Such biochemical processes might offer implications for finding possible clues to life on Mars – if Martian
soil hydromorphism or paleo-hydromorphism (i.
1992).hydrol-earth-syst-sci.
Quantification of map unit purity for different scales of soil
maps is a needed area of improvement in modern soil surveys
www. formed
Hydrol.
Hydrology has been suggested as a possible integrating
factor of soil formation and a main driving force of soil dynamic changes (Lin et al. Therefore. Earth Syst..4
Soil functions and maps: their connections to soil
hydraulic properties
Soil quality is the capacity of a soil to function within ecosystem boundaries to sustain biological productivity. To reach
a highly developed stage.net/14/25/2010/
.
Soil map is a common way to portray soil heterogeneity
and to provide soil input parameters to models. Notice the impacts
from the subsurface bedrock topography caused by weathered sandstone in (b). which can
enhance pedotransfer functions that involve soil series and
land use as carriers of soil hydraulic properties (Lin. water-dominated pedogenesis
leads to so-called soil hydromorphology (Figs. 2004). 7. but
human land use/management has significant impacts on soil
carbon and overall soil quality. 6–8) – a result of permanent or temporary state of water saturation in the
soil associated with conditions of reduction (predominantly
the accumulation or loss of Fe..1. Lin
et al. and landscape processes (Lilly and Lin. soil thickness increases as weathering progresses. This is because biological activity is often involved in soil hydromorphism on the
Earth. This is because all of the
five natural soil-forming factors affect and are affected by hydrology. Sci. 14. but also percolate through the profile and translocate weathering products (such as solutes and clays). 2009). The spatial relationships of redox depletions and redox concentrations may be used to interpret
the pattern of water and air movement in soils (Vepraskas.
showing iron transformations and the formation of ferricrete in relation to iron mobilization and water flow pathways.36
(a)
H. 2010
in ancient condition) can be found.
3. Mn. impeding further vertical percolation
of water). maintain
environmental quality.g. The concepts of “genoform”
(for genetically defined soil series) and “phenoform” (for soil
types resulting from a particular form of management in a
given genoform) (Droogers and Bouma. 25–45. the characteristics of a soil profile may reflect the total
amount water that has passed through over time. Under poorly-drained condition.
which can be quantitatively related to seasonal water table
fluctuation and frequency of soil saturation.. Soil carbon content is widely recognized as a
major factor in the overall quality and functions of soils. B horizon) would
first increase and then decrease after reaching a certain developmental stage (e. (a) A soil catena along an eroding hillslope in Australia.
3. However.. 1997) offers an interesting means to incorporate management effects into hydropedologic characterizations of soil functions. 8b. The Yellow and Grey Kandosols
(highly-weathered soils) are saturated. which is in contrast to the surface topography dominated toposequence in (a) (modified from McKenzie et al. Lin: Earth’s Critical Zone and hydropedology
Red soils
Yellow and Grey soils
(b)
Fig. (b) Freewater levels at the end of a wet season (from March to June) along
a toposequence in Australia. hydraulic and biogeochemical
properties. The presence of organic matter and a suitable temperature and pH are generally required for hydromorphism
to occur. For example. or C compounds)
(USDA-NRCS.. 1998). as illustrated in Fig. while diagenetic hydromorphism is also considerably
accelerated by microorganisms. soil genesis and
morphology would be quite different.
virtually every delineation of a soil map unit includes other
soil components or miscellaneous areas that are not identified in the name of a map unit because of limitations in map
scale and other factors (Soil Survey Division Staff. 2003).

g.1
20
1
25
0. 8. and redox features) and soil-landscape relationships (e. On the right is a hydric soil from the Coastal Plain of North Carolina. 14. (a) A general sequence of soil development from a weakly-developed Entisol (left) to a highly-weathered Ultisol (right) under welldrained conditions. Modern soil maps also need improvements for functional characterizations rather than just
for general land use planning as done in the past. where the amount of redox depletions (formed by Fe
reduction) increases with depth. the accumulation rate of organic matter (O and A horizons).2
Characteristics of hydropedology and its link to
Critical Zone science
Two general characteristics of hydropedology are suggested
below.
www. Vepraskas of North Carolina State Univ. Soil maps
can no longer be static documents. hydropedology emphasizes in situ soils in the landscape.. where T is the number of times of saturation with a period of >21 days in a year. 3. landscape hydrology. leaving
behind a light color and coarse texture E horizon and forming a high clay accumulation Bt horizon that often becomes an aquitard. which can improve the connection
between spatial soil mapping and process-based modeling. soil distribution patterns.hydrol-earth-syst-sci. and corresponds well to the average number of saturation events per year.7
30
0. Lin: Earth’s Critical Zone and hydropedology
37
(a) Well-drained soils
(b) Poorly-drained soils
Redox
Depletions (%)
15 cm sections
Saturation
Events
0
0
0
0
3
0. and D is the longest period of saturation
in a year (courtesy of M.H. (b)
Two soil profiles with typical hydromorphic features and their related monitoring data. Saturation event is calculated as
(T − 1)+(D/21). must
be generated and updated on a regular basis. Sci. 25–45. 2010
. rather. iron. Three related key aspects are
noted here:
Hydrol. catena. Precision soil mapping is of increasing demand for site-specific applications of soils information (such as precision agriculture. On the left is a hydric soil from the Mid-Atlantic
Coastal Plain in Maryland that is related to both organic matter accumulation and depletion/segregation of iron oxyhydroxides. 1998).1.6
Fig. The graduate formation of various soil horizons and the deepening of soil profile through time depend on the weathering
rate of the underlying bedrock (R horizon). tailored for a specific function or purpose. and soil map
units) are essential to understanding interactive pedologic
and hydrologic processes (Fig.. 6)... and aluminium oxides.
horizonation. and the percolation rate of water
through the soil profile.g. Further development from Alfisol to Ultisol leads to distinct eluviation of clay.
and urban development). The yearly
water table fluctuation below the soil surface and the corresponding redox potential (Eh values) at 1 m depth are illustrated (from Rabenhorst
et al.)
(Arnold and Wilding. Making connections between hydropedology and digital soil mapping is
an exciting research area. aggregation.net/14/25/2010/
3. derivative and dynamic maps. 1991). Developing quantitative
relationships between complex natural soil architecture and
soil hydrologic functions across scales is an important research area of hydropedology. which link to the two basic questions of hydropedology as described in Sect. where distinct pedogenic features (e.
First. Earth Syst.

14. 2003).g. 2008a.
Hydropedology is closely linked to CZ science because of
the key roles that soil and water play together in the CZ’s
evolution and functioning.. hence
a focus on water can provide an enhanced means for
quantifying dynamic soil functions.
– The study of palesols and palehydrology.. 2009).g. 2003.g. This requires innovative techniques for
improved quantification of soil architecture at different
scales. 2001).
greenhouse gas emission from soils. . wetlands. in quantitative ways. McClain et al. hydropedology deals with the variably unsaturated or saturated zone in the near-surface environment..net/14/25/2010/
..
including the identification of hot spots and hot moments of biogeochemical cycles in different ecosystems
(e. rather
than mapping soils without considering soil functions or
modeling soils without incorporating soil architecture
and soil-landscape patterns.
and environmental quality in general (e. 2009). Young et al. Brantley et
al.. Bouma.
which can provide a more holistic view and prediction
of subsurface flow and transport from the ground surface all the way down to the aquifer (e.
2007. Richter and Mobley. capillary fringe.. the goal of the “functionalist” is the assemblage of soil
knowledge in the form of a curve or an equation. 2000). 2009).
and subaqueous soils (soils formed in sediment found in shallow permanently flooded environments such as in an estuary)
(Demas and Rabenhorst. Lam et al. Lohse et al. 2005).g.
– The connection between hydropedology and land use
planning...g. 25–45.
– Hydropedology attempts to link the form and function
of a soil system across scales (Lin et al. .. Key approaches or steps that need to be taken
www. patterns.. and remote sensing of soil climate (e. Three related key aspects are
noted here:
– Hydrology has the potential to be an integrating factor
for quantifying soil formation and evolution as well as
for understanding soil changes (Lin et al.g.” Such a union of soil maps and soil functions is what
hydropedology hopes to accomplish.
including the shallow root zone.
temporally-saturated soil zone.hydrol-earth-syst-sci. a crushed or pulverized
sample of the soil is related to the soil formed by nature like a pile of debris is to a demolished building. 2007. Sci. 2006). In contrast. Li et al. Specifically. In this section.
4
Opportunities for advancing Critical Zone science
and hydropedology
Given the growing interests in CZ science (e.
Second.
– The interpretation and quantification of soils as
historical records of environmental changes could
be improved if hydrologic data are considered
simultaneously. 2006a)... 2007). Li et al. ecological health and diversity.
– Hydropedology considers the soil in the real world as a
“living” entity in the landscape.. 2009).
– The integration of hydropedology and hydrogeology. 2006a. 2009) and hydropedology
(e. which includes issues related to soil moisture
and global climate change. 2000). such as delineating
hydropedologic functional units as soil-landscape units
with similar pedologic and hydrologic functions (Lin et
al..
ground water recharge. Lin et al..
As Kubiena (1938) pointed out. Lin: Earth’s Critical Zone and hydropedology
– Hydropedology calls for a new era of soils research that
is based on soil architecture (rather than solely on soil
texture) so that the prediction of flow and reaction pathways. three approaches are
discussed.
– The linkage between hydropedology and hydrometeorology. 2008b).. these links include
the following:
– The interrelationships between hydropedology and ecohydrology.
– New ways of characterizing and mapping soils could be
improved by linking to hydrology.. because how natural soils “throb” upon precipitation inputs under various climates offers clues as
to “what” can best be done and “where” with the lowest risks and the greatest opportunities for land use and
management (e. especially in situ noninvasively. This has been demonstrated in some
palesol and palehydrology studies (e.
it is the union of the geographic and the functional method
that provides the most effective means of pedological research.. Clearly. and residence times can be made more
realistically. 2008) and
other related efforts around the world (Richter and Mobley. it is beneficial at this early stage of their developments to discuss some
possible ways forward. Earth Syst. Ashley and Driese... which shows
valuable historical records of the past environment
and ancient landscape-soil-water-ecosystem-climate relationships (e. and then linking
such soil architectural parameters to field-measured soil
hydraulic properties. Ashley and
Driese.. which are based on recent efforts in Critical Zone
Observatories (CZOs) in the US (Anderson et al.
Lin (2007) also suggested that a crushed soil sample is
as akin to a natural soil profile as a package of ground
beef is to a living cow.38
H. not a “dead” material. 2010
– The coupling of hydropedology and biogeochemistry. Lin.g.
Jenny (1941) noted. and how they influence soil moisture.g.g. deeper vadose zone. soil carbon sequestration..
Hydrol. “The goal of soil geographer is the assemblage of soil knowledge in the form of a map.

5) deserves particular attention. continental and global) levels.”
The four general characteristics of the CZ discussed in
Sect. However. 2006.
2.. . meaning that the systems properties cannot be revealed by a reduction to some
observations of the behavior of their components. 1 to 5. many other
disciplines are involved in CZ studies (e.
Integration of disciplinary research is a key to future
progress in CZ science. watershed and regional). Instead. general systems
theory. Besides hydropedology. The role of these interfaces in understanding
the interactions and feedbacks between compartments
and their regulatory impacts on the whole CZ deserves
more attention in defining research questions for the CZ
(H. which appears to be the most challenging but also the most easily
neglected. and complexity science to help develop an evolutionary and holistic worldview of the soil and the CZ (which goes
beyond the classical mechanistic and reductionistic worldview). Entropy
has the intrinsic properties of time irreversibility as well as
quality and information that other thermodynamic functions
lack (Tiezzi. leading to possible “hot spots” and “hot moments”
in the CZ (McClain et al. biological.
No attempt. 2. 2009. Toffler (1984) remarked.. Vereecken.hydrol-earth-syst-sci.
“Conservation without evolution is death. Li et al.g. Scaling: Scale bridging remains a huge challenge in
nearly all hydrologic. we often forget to put the pieces back together again . 25–45. The CZ’s general characteristics.. 2). Key
interfaces in the CZ provide fertile ground for transformative research. molecular and pore). to mesoscopic (e.
in addition to the conservation of energy and mass (Fig. pedon
and catena). 3). and
ecological studies. Sci. is made here to exclusively consider
all possible approaches or steps.net/14/25/2010/
39
2006. Ulanowicz.
4. Lin: Earth’s Critical Zone and hydropedology
or strengthened in order to achieve significant advancements
in our understanding and prediction of the CZ are explored. Coupling: Biogeochemical and ecological cycles are
tightly coupled with the hydrologic cycle and the soil
reservoir. 2010
. 2010b). It is highly desirable to clarify
how dominant processes and their controls change with
spatial and temporal scales. personal communication..g.
as summarized in Fig. the soil science community is
embracing land use/management impacts on soils as a
new frontier (Richter. We say ceteris
paribus–all other things being equal. 3).
Entropy. pedologic.g. We are good at it. In particular.g.
with a focus on those aspects germane to hydropedology. and emergent properties exist at the systems level that are beyond the individual components’ properties. and to explore quantitative means to transfer knowledge from microscopic
(e.
and to megascopic (e. Forcing: Traditional geosciences have viewed humans
and life processes as external drivers rather than as intrinsic parts of the whole system.” These words of Bateson (1979)
underline a fundamental characteristic of complex natural
systems such as soils and the CZ. where interactions among physical.
chemical. Tiezzi (2006) suggested the need for
a new theory to describe ecosystem’s behavior.
The intermediate scale associated with medium number
systems (Fig.g. as a core thermodynamic variable.H. Evolution without
conservation is madness.. .1
Integrated systems approach
Most natural systems are irreducible. 2003). 2007. Jørgensen et al.. In this way we can
ignore the complex interactions between our problem and the
rest of the universe. 3. Such a discussion hopefully can
stimulate more synergistic efforts in the community. 2009). 14. Interfacing: The presence of interfaces between two or
more compartments is an important element of the CZ
(Fig. Reductionist’s
approach does not work well for revealing the complexity in
soils and the CZ because of evolutionary nature. The hydrologic community is now keenly interested in human
impacts and biological forcing in the hydrologic cycle (CUAHSI. “One of the most
highly developed skills in contemporary Western civilization
is dissection: the split-up of problems into their smallest possible components.. 2007).
4. as
alluded to in Figs. 2004).
www. the geochemical community is
tackling weathering that includes anthropogenic forcing
(Anderson et al. Richter and Mobley.. and anthropogenic processes prevail. 2007.
3. it is these
forces that have driven the CZ far away from equilibrium. 2009). The classical
average budget-based cyclic approach needs to be complemented by more process-based dynamic approach to
quantify enormous variability across space and time. macroscopic (e. however. offers a possible new perspective to understand the interactions between
soil systems and their surrounding environment..
Hydrol. biogeochemical. 2009). Lin (2010b) emphasized the accumulation of entropy and information in the evolution of the CZ. The examples given below
are meant to be illustrative. Brantley et al. cumulative
effects. 2007).2 offer a holistic framework for a systems-based understanding of the CZ (Fig. Lin (2010b) further discussed
the use of non-equilibrium thermodynamics. because the
laws that describe the system may be qualitatively different
from those that govern its individual units.. imply the following four grand challenges for advancing CZ science that demand an integrated
systems approach:
1.. and provided the CZ with chances and choices
in its evolutionary process (Lin. So good. Earth Syst. Reconciliation of the geological “big” cycle
and the biological “small” cycle is essential for understanding and predicting the CZ (Fig. and constant interactions and feedback loops among
individual units. 4).
it is necessary to observe the entire system to capture its behavior because everything is dependent on everything else
by direct or indirect linkages.

2010a. validates model outputs. but irreversible
and threshold-like in the long-term. monitoring. This evolutionary process presents significant challenges to designing and implementing scientific monitoring program for the CZ. For
example. and modeling (3M)
can provide a possible integrated and evolutionary approach
to address the complexity and dynamics in the CZ (Fig. 14.net/14/25/2010/
. however. and scenario prediction. would hinge on technological breakthrough in characterizing subsurface networks (Lin. Shaded relief map at the top shows the locations of the Critical Zone
Observatories (CZOs) funded in the US (red stars indicate the CZOs funded in 2007. subsurface flow networks are often embedded in
land mosaics. Such an evolutionary approach allows the
development of adaptive strategy and the refinement of models and monitoring networks as knowledge and database are
accumulated...20
0. 2006.g. liquid. biodiversity. and supplies ground truthing for remote sensing. 2007..2
Evolutionary mapping-monitoring-modeling
framework
An iterative loop of mapping.. 2007.
www. the hydrology
community is awaiting a conceptual breakthrough that can
go beyond the classical small-scale physics (Beven. it is inadequate to determine soil change by only one characteristic.
knowledge integration. When integrated with real-time monitoring
and spatially-distributed mapping. Therefore. Considering the multi-phase nature of the soil
system (gaseous.
Kirchner.
2010a). solid... and biotic phases).05
R(37cm)
10
A(8cm)
5
A-CR(10cm)
0. Models can
also guide the design and site selection for monitoring and
ground truthing.
Such an approach.40
H. long-term monitoring of soil
changes is necessary. monitoring.hydrol-earth-syst-sci. Tromp Van Meerveld and McDonnell.
4. b). and white stars indicate the CZOs funded in 2009. Lin. 2006.
modified from Anderson et al.
Monitoring is essential to record temporal dynamics and
coupled cycles in the CZ. 25–45.25
15
0. and global environmental changes all depend on the
evolution of the soil. However.30
20
0. 2008). Lin et al. 2006. Iterative loop of mapping.10
0. sustainable land management. Some soil changes are inherently longterm. Sci. undetectable in a short period of time. 2010
ecosystem services. 9. which may be conceptualized to provide an alternative to modeling coupled hydrologic and biogeochemical processes.
Models are useful tools for quantitative assessment. 2008b). models can better address
temporal trends and spatial patterns.00
9/23/06
Precipitation (mm/10-min)
Monitoring
Volumetric water content (m3/m3)
Local
0
10/23/06
11/22/06
Fig.
Hydrol. Lehmann et al.
because each soil phase and property has its own characteristic response time. and modeling (3M) as an integrated and evolutionary approach to address the complexity and
dynamics of the Critical Zone across scales and geographic regions. McDonnell et al.
perhaps as a good starting point to deal with many uncertainties involved. It also provides model inputs. where internal network structures can govern
vertical and lateral preferential flow dynamics and thresholdlike hydrologic response (e.15
0. Lin: Earth’s Critical Zone and hydropedology
Mapping
Puerto
Rico
Global
Regional
Modeling
0. 9). However. Earth Syst.

the EU 7th
Framework Programme (http://cordis.H. An integrated network for observing. A key to connect these two is the fabric of the
subsurface over the landscape that should be mapped at appropriate spatial and temporal resolution so that meaningful extrapolation and upscaling of point-based data could be
made. This is why we need the mapping of
the subsurface heterogeneity.. 2009). Sci. Then. and deeper insights –
may be gained (Anderson et al. modeling. a global
alliance is suggested here. Furthermore.
Significant technological advancements are needed in
many aspects of CZ science and hydropedology. In addition. But the signals of
two sensors at nearby locations in many soils may be completely uncorrelated.
Inherent heterogeneity of spatial structures across scales
remains a major challenge in understanding the CZ.
4. In
addition. a synergistic effort to
foster a global alliance for monitoring.eu/fp7/) has selected a project for funding four CZOs in EU (S. mapping.nsf. the spatial resolution. In 2009. Geostatistical functions should be derived from landscape stratified units (such as soil type. of the CZ across scales and geographic regions (Fig. 9). 2008). our capability to predict the behavior and evolution of the CZ in
response to changing environment can be improved significantly – if such a global alliance can be fostered effectively.europa. Therefore.
– The need for advanced subsurface characterization technologies.3
Fostering a global alliance
With growing interest in international scientific communities to establish various environmental observatory networks
and to address “big” science questions. The
solid land is unlike the atmosphere and the ocean (which
have much greater intermix and can be modeled as a continuous fluid). 2009). the land poses hierarchical heterogeneities
with controlling structures that are different at various scales. Long-term monitoring.net/14/25/2010/
41
– The need for online databases that are open to the community and are comprehensive with visualization and
analysis toolboxes for extracting useful information out
of these databases effectively. rather. the US National Science Foundation funded
three Critical Zone Observatories (CZOs) (Fig. This is a fundamental difference compared to
atmospheric monitoring where the heterogeneity of the system can be explored at one sensor location. or depth penetration of many geophysical and remote sensing technologies remain limited for
high-resolution investigations of belowground CZ. Mapping is also a prerequisite for
spatially-distributed modeling. Currently. While geostatistics provides
powerful interpolative tools after a dataset has been gathered on a particular area. and modeling of the CZ is desirable. In 2009. which may be best realized in a coordinated way to maximize the benefit for global environmental
research. interests have also emerged in the
community to forge independently conceived observatories
into a network from which broader understanding – larger
spatial scales. 9)
can serve many purposes of societal importance. we
can use the knowledge of soil formation and the known interrelations of soils within landscapes to establish a mapping
strategy that is supported by a soil-landscape modeling.
Flow networks and their temporal changes are as important
as flow rates in the three-dimensional heterogeneous landscape.
Hydrol. and only then. No one team
or organization can do that alone. Mapping also provides a means of diagnosing and
stratifying the landscape for determining optimal location
and number of monitoring sites and for designing meaningful model experiments. Together. now is the right time to foster a global alliance for
studying the CZ. 2010
. 2003.. and a diversity of funding
sources supporting a heterogeneous mixture of overlapping
programs is probably the best formula for long-term stability
of observatory networks (Keeling.
Lin. geology. it is not a powerful extrapolative
tool. personal communication. Banwart. 2010a. 25–45.-J. Therefore.
In 2007. which is typically unknown or hard to quantify
with currently available technologies (Vogel and Roth. 9) “that
will operate at the watershed scale and that will significantly advance our understanding of the integration
and coupling of Earth surface processes as mediated by
the presence and flux of fresh water” (http://www. but it is clear that it will require
inputs from many basic and applied disciplines. Ground-based monitoring generally collects
point-based data. Vogel. In the
meantime.
www. cross-site comparisons.gov/
pubs/2006/nsf06588/nsf06588. Earth Syst. 2008). All
processes within the subsurface are bound to this structural
framework.net/). Similar efforts are also being pursued in some other countries around the globe. and sustaining the Earth’s CZ as a whole
(rather than individual compartments of the CZ) is still in its
early stages of development.
personal communication. b). three additional CZOs are selected for funding (Fig. or temporal frequency. land use)
and not indiscriminately across a broad landscape without
prior partitioning of the causes of variability. 14. such as:
– The need for new and improved sensors that are reliable
in all seasons and robust for long-term monitoring. Optimization of whole systems for multiple benefits rather than one
benefit permit synergistic outcomes and would be more costeffective in the long-run. Lin: Earth’s Critical Zone and hydropedology
The value of mapping in the study of the CZ should not
be overlooked.
which dictate the pathways and patterns of flow and reaction.hydrol-earth-syst-sci. while modeling often attempts to cover
large areas. can the
point-based monitoring provide the required observations to
develop and improve predictive potential of process-based
models (H. along
with precision spatial mapping and process-oriented modeling.pdf). many CZ properties can not yet be monitored
in situ with a real-time sensor. German Helmholtz Association has established
four TERestrial ENvironmental Observatories (TERENO) in
2008–09 to investigate the consequences of global change
for terrestrial ecosystems and the associated socioeconomic
implications (http://www.tereno.

International Institute for Applied Systems
Analysis. The hydrologic cycle.
Acknowledgements. Sediment Res. and how the hydrologic cycle feedbacks to pedogenesis and soil functions. O.. Olduvai Gorge. S.
Ashley. Brantley.: Mind and Nature: A Necessary Unity. the famous “Keeling Curve” of long-term CO2 data has
demonstrated the value of continuous recording of a seemingly routine atmospheric measurement. in: Spatial variabilities of soils and landforms. J. 1991.
Bateson. Am. 265–272. Kump. P. John Maximilian K¨ohne is thanked for translating
the paper of D. Some of the key approaches or steps
for achieving significant advancements may include (but not
limited to):
– Iintegrated systems approach that cross-fertilizes the
principle of energy and mass conservation with the principle of entropy and information accumulation in the
evolutionary. Sci. Hydropedology is concerned with how the
subsurface heterogeneity develops and evolves. Blum. have become
prominent forces of changes in the soil and the CZ (at least
at the human time scale). Hering. J. S. Richter... 1974). S. Likens and Bormann. which turned out
to be a vital sign of the Earth’s climate and led to the first
alert to the world about the anthropogenic contribution to
the “greenhouse effect” and global warming (Keeling. how soil
distribution pattern influences hillslope and watershed hydrology. carbon content. J. C. and Duffy. David Chittleborough. temperature. T. V. Earth Syst.
Arnold. and Targulian. longlasting air and water quality.
– Formation of a global alliance that monitors. G. Drever. together with human activities. 85. 1972.
Anderson.: Critical zone observatories: Building a network to advance interdisciplinary study
of Earth surface processes. M. A. 70..
5 Summary and conclusion
The emerging interests in the CZ and the establishment of
CZOs and alike in different parts of the world provide an excellent opportunity to advance the understanding and management of the most complex and heterogeneous biomaterials on the Earth surface. and Driese. SSSA Special Pub. John
Wiley & Sons. and Wilding. EOS. 1990. 25–45.
Soil Sci. Soc. 2008). is essential to the sustainability of ecosystem services. 14.. Chadwick. Harry Vereecken)
and the handling editor Hans-J¨org Vogel helped improve the quality
of this paper.-J. L. and deeper insights into
the CZ. S. R.: Paleopedology and paleohydrology of a volcaniclastic paleosol interval: Implications for early
pleistocene stratigraphy and paleoclimate record. For example. P. and Wilding.: Proposed initiative would study Earth’s
weathering engine. Madison.
Hydrol. W. respiration..
and Jack Watson..net/14/25/2010/
.
Tanzania.. L. and thus deserve elevated attention
in the integrated studies of the CZ. Chichester.... P. multiscale. J. Report of an IIASA-ISSS-UNEP task force on the role of
soil in global change. J. and White. continuous CZ observations are
essential as they have the potential to open our eyes for unexpected but relevant developments and processes.: The need to quantify spatial variability. 2010
Hydropedology is an important contributor to CZ science
through addressing the interface between the pedosphere and
the hydrosphere. Laxenburg. G. E. 73–120.. edited by:
Mausbach. 1979.. and interdisciplinary studies of
the complex CZ. 28. while the soil is at the central junction of various interacting compartments in the CZ. G. The author acknowledges the support from the
US National Science Foundation through the Shale Hills Critical
Zone Observatory grant (EAR-0725019).
D. Peer review comments provided by
anonymous reviewers (one not so anonymous. R..
– Evolutionary mapping-monitoring-modeling framework that allows the development of adaptive strategy
and systematic characterization of subsurface heterogeneity across scales. maps. the CZ as a whole
is much broader than just soils.. evolutionary
treatment of complex systems. L.. R. Dutton. J. A broadened perspective is
therefore needed to propel soil science forward and to better
integrate its knowledge base with other bio. 2008. Lin: Earth’s Critical Zone and hydropedology
Besides spatial networking of CZOs and alike. M... However.. Chris Graham. Inc. Vogel
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